A conventional gas turbine engine includes a compressor, a combustor and a turbine. The compressor compresses ambient air which is supplied to the combustor where the compressed air is combined with a fuel and ignites the mixture, creating combustion products defining a working gas. The working gas is supplied to the turbine where the gas passes through a plurality of paired rows of stationary vanes and rotating blades. The rotating blades are coupled to a rotor and disc assembly. As the working gas expands through the turbine, the working gas causes the blades, and therefore the rotor and disc assembly, to rotate.
Combustors often operate at high temperatures that may exceed 2,500 degrees Fahrenheit. Typical turbine combustor configurations expose turbine blade assemblies to these high temperatures. As a result, turbine blades must be made of materials capable of withstanding such high temperatures. In addition, turbine blades often contain cooling systems for prolonging the life of the blades and reducing the likelihood of failure as a result of excessive temperatures.
Typically, turbine blades comprise a root, a platform and an airfoil that extends outwardly from the platform. The airfoil ordinarily comprises a tip, leading edge and a trailing edge. Most blades typically contain internal cooling channels forming a cooling system. The cooling channels in the blades may receive air from the compressor of the turbine engine and pass the air through the blade. The cooling channels often include multiple flow paths that are designed to maintain the turbine blade at a relatively uniform temperature. However, centrifugal forces and air flow at boundary layers often prevent some areas of the turbine blade from being adequately cooled, which results in the formation of localized hot spots. Localized hot spots, depending on their location, can reduce the useful life of a turbine blade and can damage a turbine blade to an extent necessitating replacement of the blade.
Operation of a turbine engine results in high stresses being generated in numerous areas of a turbine blade. One particular area of high stress is found in the airfoil trailing edge, which is a portion of the airfoil forming a relatively thin edge that is generally orthogonal to the flow of gases past the blade and is on the downstream side of the airfoil. Because the trailing edge is relatively thin and an area prone to development of high stresses during operation, the trailing edge is highly susceptible to formation of cracks which may lead to failure of the airfoil.
A conventional cooling system in the airfoil of a turbine blade assembly may include cooling fluid passages to provide convection cooling in the airfoil trailing edge, and discharge a substantial portion of the cooling air through the trailing edge of the airfoil. For example, a typical trailing edge cooling configuration may comprise generally constant diameter cooling passages provided with pin fins extending transversely across the passages to increase the convective cooling in the trailing edge. As a result of this configuration for cooling, a thicker trailing edge is typically required in order to accommodate the passages. In some turbine stage blading designs, a larger trailing edge thickness may induce high blockage and thus reduce the stage performance.
Hence, the size and space limitations make the trailing edge of gas turbine airfoils one of the most difficult areas to cool. In another known configuration, the trailing edge comprises an overhang where the suction sidewall extends further downstream than the pressure sidewall. In such a configuration, the pressure side includes slots for cooling fluid to exit from cooling passages and provide pressure side bleed for the airfoil trailing edge cooling. For example, this type of configuration commonly includes an entrance length having a constant cross sectional area, followed by an expansion in the transverse direction extending between the pressure and suction sidewalls and a constant dimension in the spanwise direction. Subsequently, at a cooling slot breakout defined at the trailing edge overhang, the channel may have an expanded section in the spanwise direction. This type of cooling concept is effective to reduce the airfoil trailing edge thickness, but results in shear mixing between the cooling fluid and the mainstream flow as the cooling fluid exits from the airfoil pressure side. The shear mixing of the cooling fluid with the mainstream flow reduces the cooling effectiveness in the area of the trailing edge overhang and thus may result in an overtemperature condition on the suction side of the airfoil. Frequently, the deterioration of trailing edge overhang area due to overtemperature conditions becomes a limiting condition for the service life of the entire airfoil.